This invention relates to a device for supporting a temperature sensor in the interior of a liquid, to a temperature sensor assembly incorporating such a device and to a method of determining the temperature of a liquid; more especially the invention is concerned with such a device, sensor assembly and method in which the temperature sensor is an immersion pyrometer for measuring the temperature of a molten metal, molten salt or other high temperature liquid.
In particular the invention is concerned with such a device, sensor assembly and method in which the device supports a hot thermocouple element, and the device floats in the liquid, whereby the hot thermocouple element may be disposed at a predetermined, desired or selected position within the liquid.
Immersion pyrometers for measuring the temperature of a molten metal or other high temperature liquid are typically immobile and are located at a fixed position in the liquid. For example, the pyrometer may extend into the liquid through a wall of a vessel holding the liquid.
The immersion pyrometer is housed or supported in a device which protects the pyrometer from the liquid. Examples of protective devices are described in U.S. Pat. No. 5,474,618 C. Allaire; U.S. Pat. No. 5,577,841 Cowall; U.S. Pat. No. 4,692,556 T. Bollen et al and U.S. Pat. No. 5,456,761 M. Auger et al.
Since the immersion pyrometer is immobile, different parts of the pyrometer are exposed to the liquid as the level of liquid rises or falls in the vessel in which it is housed. Likewise, in the case where the liquid is a molten metal, and a slag is formed on the surface of the molten metal, rise and fall of the level of molten metal results in different parts of the pyrometer being exposed to the slag. In addition the portion of the pyrometer not immersed in the liquid and which is exposed to possible attack by the atmosphere above the liquid, varies with the rise and fall of the liquid. A further problem is that these prior pyrometers are subject to a fluctuating thermal gradient in the liquid, as well as a fluctuating liquid line level.
It is an object of this invention to provide a device for supporting a temperature sensor such as a thermocouple element for determination of the temperature of a liquid, which floats in the liquid.
It is a further object of the invention to provide a temperature sensor assembly employing a device for supporting a temperature sensor of the assembly, which device floats in a liquid, the temperature of which is to be detected.
It is yet another object of the invention to provide a method of determining the temperature of a liquid in which a device supporting a temperature sensor floats in the liquid with the temperature sensor disposed beneath the upper surface of the liquid.
According to the invention there is provided a device for supporting a temperature sensor within the interior of a liquid comprising: a sheath having an interior cavity extending from an open end to a closed end for receiving the temperature sensor, said device being adapted to float in the liquid with said closed end immersed in the liquid.
According to another aspect of the invention there is provided a temperature sensor assembly for determining the temperature of a liquid comprising: a) a temperature sensor, b) a device for housing the temperature sensor for supporting the sensor within the interior of the liquid, said device comprising a sheath having an interior cavity extending from an open end to a closed end, said temperature sensor being housed within said cavity, said device being adapted to float in the liquid with said closed end immersed in the liquid.
According to yet another aspect of the invention there is provided a method of determining the temperature of a liquid comprising: providing a bath of liquid having an upper surface, floating in said liquid a device supporting a temperature sensor, said device comprising a sheath having an interior cavity extending from an open end to a closed end, said temperature sensor being housed in said cavity at said closed end, and said closed end being immersed in said liquid, allowing the temperature of the sensor to adjust in response to the temperature of the liquid, and determining the temperature of the liquid from the adjusted sensor.
The invention is applicable to sensing the temperature of liquids generally, but has particular application in the sensing of the temperature of materials which are solid at normal temperatures and liquid at elevated temperatures.
In preferred embodiments the invention relates to the sensing of the temperature of molten metals and salts and is more particularly described hereinafter by reference to an especially important embodiment in which the temperature of a molten metal is to be determined.
i) Supporting Device
The supporting device of the invention, which supports a temperature sensor, for example, a hot thermocouple element, is constructed of materials and/or has design parameters such that it floats in the molten metal, with a lower portion of the device, housing the hot thermocouple element, immersed in the molten metal. In this way an upper end of the device extends above the upper surface of the molten metal, and a lower end of the device extends below the molten metal surface.
Alternatively if the device is not of a material or constructed with parameters such that it floats in the molten metal, it may employ a separate component or member which renders it floatable.
The sheath is suitably formed of a refractory material which will withstand the molten metal, and retain its structural integrity when floating in the molten metal with its lower end immersed in the molten metal.
The sheath has an internal cavity defined by an elongate bore which is closed at the lower end of the sheath and open at the upper end of the sheath.
A hot thermocouple element and its connecting lead may thus be inserted along the cavity to locate the hot thermocouple element at the closed end of the bore.
The refractory sheath thus protects the hot thermocouple element and its connecting lead. The refractory material may be a mixture of Al2O3, SiO2, CaO, MgO, ZrO2, AlN, SiC, Si3N4, C and the like. The selected composition is such that it confers to the refractory sheath the required resistance to corrosion by the liquid whose temperature is to be measured, as well as the required thermomechanical properties, including mechanical strength and thermal shock resistance.
The device also includes an outer protective shield surrounding the refractory sheath, for example, a metallic shield and particular reference will be made hereinafter to a metallic shield. This metallic shield is suitably in the form of a cylinder with one open end (toward the bottom part of the device) and one closed end (toward the top part of the device) through which is inserted the thermocouple element and its lead. The length of this metallic shield is at least equal to the length of the non-immersed part of the refractory shield after the later has been introduced into the liquid whose temperature is to be measured.
The metallic shield is suitably made of a pure metal or an alloy whose melting temperature should be higher than the temperature of the atmosphere located above the liquid whose temperature is being measured. If the atmosphere is oxidizing, the use of a metal or alloy having a high resistance to oxidation is preferred, such as Nixe2x80x94Cr or Nixe2x80x94Co alloys.
The inner diameter of the metallic shield is greater than the outer diameter of the refractory sheath. This creates a gap between the shield and the sheath. This gap, whose width or thickness is preferably less than 5 mm, is closed at one end by the closed end of the metallic shield, and at the other end by the liquid in which the device is immersed.
The metallic shield provides a number of functions including:
a) It prevents the deterioration of the refractory sheath by the action of the atmosphere above the liquid level;
b) It minimizes the corrosion of the refractory sheath by the action of slag or skim present on the upper surface of the liquid;
c) It minimizes the thermal shock effect on the refractory sheath during its insertion into the liquid.
For example, refractory sheath made of materials such as Al2O3/C, MgO/C or ZrO2/C, are protected by the metallic shield against oxidation from the oxidizing atmosphere encountered above molten steel in a tundish. The sealed gap between the shield and the sheath becomes rapidly filled with reducing gas, during use, after the oxygen contained initially in the gap has been converted into carbon monoxide by reaction with the carbon contained inside the refractory materials. In this same example, the molten slag which is formed on the upper surface of the molten metal in the tundish, is prevented from reaching the refractory sheath by the presence of the metallic shield whose resistance to dissolution by the slag is much higher. When the device is immersed into the molten steel, the immersed lower end of the refractory sheath is exposed rapidly to a temperature of about 1500-1600xc2x0 C. while the non-immersed upper end is exposed to a maximum temperature of less than about 800xc2x0 C. With the presence of the surrounding metallic shield, which is highly thermally conductive as compared to the refractory sheath, the initial thermal gradient in the refractory sheath near the upper surface of the metal is reduced since the heat is conducted more rapidly toward the top of the shield.
The refractory sheath should have sufficient heat conductivity to permit the thermocouple element which is houses, to respond to the temperature of the molten metal.
In especially preferred embodiments the device is adapted to float with the closed or lower end disposed at a predetermined or selected distance below the upper surface of the liquid, for example, molten metal, in which the device floats. In this way the thermocouple element may be disposed at a desired, predetermined or selected distance below the upper surface of the liquid, so that the temperature at a desired location within the liquid can be determined.
Thus in a particularly preferred embodiment of the invention there is provided a device for supporting a thermocouple lead within the interior of a liquid the temperature of which is to be determined comprising: i) a refractory housing having an interior cavity for receiving a thermocouple lead, said housing having an upper end and a lower end, ii) an outer protective shield surrounding at least the upper end of said refractory housing, iii) means to render said device floatable in the liquid with said upper end of said refractory housing extending above the liquid, and said lower end of said refractory housing immersed in the liquid, and with said outer protective shield shielding said upper end of said refractory housing from an environment above the liquid, and iv) said refractory housing being adapted to isolate the thermocouple lead from the liquid.
ii) Temperature Sensor
The temperature sensor assembly comprises a supporting device as described hereinbefore, and a temperature sensor. In particular the temperature sensor may comprise the hot thermocouple element of a thermocouple, the hot thermocouple element being disposed in the bore of the sheath, adjacent the closed or lower end and the cold thermocouple element and related components of the thermocouple being located remote from the device and the molten metal.
As is well known the hot thermocouple element is electrically connected to the cold thermocouple element, and the potential difference between the elements, arising from their difference in temperature is measured by conventional means. The potential difference provides a parameter from which the temperature of the liquid can be determined.
The thermocouple may be made of conventional thermoelectric elements, such as Pt/Ptxe2x80x94Rh metals or the like, which are usually inserted inside a protective ceramic or metal shield, such as alumina or Nixe2x80x94Cr or Nixe2x80x94Co alloys or the like.
The hot junction of the thermocouple element is suitably in contact with the closed end of the refractory sheath.
Thus in an especially preferred embodiment of the invention there is provided a thermocouple assembly for determining the temperature of a liquid comprising: a) a thermocouple having a hot thermocouple element and a cold thermocouple element and means to determine a potential difference between said elements, b) a device supporting said hot thermocouple element within the interior of a liquid the temperature of which is to be determined, said device comprising: i) a refractory housing having an interior cavity for receiving said hot thermocouple element, said housing having an upper end and a lower end, ii) an outer protective shield surrounding at least the upper end of said refractory housing, and iii) means to render said device floatable in the liquid with said upper end of said refractory housing extending above the liquid, and said lower end of said refractory housing immersed in the liquid, and with said outer protective shield shielding said upper end of said refractory housing from an environment above the liquid, and iv) said refractory housing being adapted to isolate the hot thermocouple element from the liquid.
iii) Method of Determining Temperature
In the method of the invention the supporting device housing the temperature sensor floats in the molten metal. In this way the temperature sensor may be located at a desired, predetermined or selected distance below the upper surface of the molten metal, in a molten metal bath.
Even though the level of molten metal and thus the location of the upper surface of the molten metal, may rise and fall, the floating device will maintain the temperature at the same distance below the upper surface of the molten metal.
Thus the temperature sensor may be maintained at a constant distance from the rising and falling metal surface, within the metal bath, and the temperature at a desired location within the metal bath may thus be monitored continuously over a long period.
Thus in a particular embodiment of the invention there is provided a method of monitoring the temperature of a liquid comprising: A) providing a thermocouple assembly having a hot thermocouple element, a cold thermocouple element and means to determine a potential difference between said elements. B) isolating said hot thermocouple element within an interior cavity of a refractory housing having an upper end and a closed lower end, at least said upper end being surrounded by an outer protective shield, said refractory housing and said outer protective shield defining a flotation device, C) floating said device in said liquid with said lower end of said refractory housing immersed in said liquid and said upper end extending above said liquid such that said outer protective shield shields said upper end from an environment above the liquid, D) allowing the temperature of the hot thermocouple element to adjust in response to the temperature of the surrounding liquid, and E) measuring the potential difference between said hot and cold elements at the cold ends of the leads therefrom and determining the temperature of the liquid therefrom.